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Matlab Gain Margin problem.

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reik

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For the Matlab bode diagram, normally when we want to obtain the gain margin, we have to cross the line from phase -180 and touch the curve. (Picture1 is what I did) however from the matlab, the gain margin crossover is start from -540 instead of -180 degree by using "Margin" function(Picture2 is what Matlab shown).

Attached is the problem's picture, may I know should I stick with -180 like the "picture1" ? or just follow what the Matlab show which is -540?
picture1.png
Picture2.png
 

Looks rather strange. The gain drops with app. 20 dB/dec and the phase goes rapidly down (rather than to approach 90 deg.) . Something must be wrong in your simulation setup.
 

Below is my code, its just simple but wonder what is the problem =(


num=[24];
den=[11.5 1];
g=tf(num,den);
set(g,'inputdelay',2.3)
bode(g)
margin(g)
Click to expand...


---------- Post added at 19:49 ---------- Previous post was at 19:36 ----------

What i guess is, due to the gain margin is positive, thus the system auto change to -540 so the gain margin will be negative? Does this make sense?
 

reik said:
What i guess is, due to the gain margin is positive, thus the system auto change to -540 so the gain margin will be negative? Does this make sense?
Click to expand...

No, as I told you: The phase response looks strange. Check your simulation setup.
I suppose, you have shown the loop gain - correct?
 

sorry i dont understand...because this question is come from my lecturer, assume the question is correct, is it logic which the matlab mark the curve at -540 degree?
 

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